The present invention relates to improvements in extended nip presses pressing water from a traveling fibrous web wherein the nip is formed by a sliding shoe having a hydraulic film of lubricating fluid between the shoe and the belt and more particularly, the invention relates to improvements in controlling the lubricating fluid for removing it from the belt downstream of the shoe and preventing the fluid from being carried along with the belt and preventing it from migrating around the edge of the belt onto the web side.
In a conventional paper making machine, after the web is formed, it is carried through a press section where the water is mechancially expressed from the fibrous web. Improvements in press sections have changed from the conventional two roll press to what has been known as an extended nip press wherein the web is subjected to a continuing pressure for a longer period of time than with the simple two roll press. Developments in these extended nip presses have included a roll as one of the pressing members with the other pressing member being a continuous impervious belt pressed toward the roll by an arcuate sliding shoe which develops a film of hydraulic lubricant between the belt and the shoe to eliminate friction and help aid in developing uniform pressure completely across the pressing zone through which the web passes. Examples of these improved sliding shoe presses are shown in U.S. Pat. No. 3,783,097, E. J. Justus and an application copending herewith, Ser. No. 939,449, Mohr et al.
The lubricating fluid which is delivered to form the hydraulic film between the shoe and traveling belt must be uniformly delivered across the web and in one form of mechanism, is provided by a series of nozzles arranged and controlled so that they deliver a lubricating fluid such as oil to the leading edge of the shoe which is relieved and forms a uniform hydraulic film completely across the shoe. As this film is formed, lubricant adheres to the belt and travels along with the belt trailing out from behind the shoe. This lubricating oil must then be controlled so that it does not continue to travel along on the surface of the belt so as to be compressed or fly off of the belt surface as the belt is carried over guide rolls. Further, the lubricating oil must be controlled so that it does not migrate toward the edge of the belt and pass over the edge where it will fly out into the surrounding atmosphere or will pass out over the edge of the belt and travel onto the web side of the belt so as to contaminate the web. The web is carried against a felt or between two felts, and these felts must be maintained to receive the water expressed from the web and satisfactory operation dictates that the lubricant cannot get into the felts to affect their water receptivity and to contaminate the web. Various means have been attempted to control and remove lubricant from the surface of the belt, but problems are presented with a belt that is traveling at speeds of 300 to 5,000 feet per minute. Further, the removal must be effected uniformly across the surface so that lubricant is not continued to be carried in streaks or ridges along with the belt so as to possibly return on the belt surface into the nip between the belt and the shoe to adversely affect the uniform pressure which must be maintained in the hydraulic film between the shoe and the belt. Further, complete removal particularly along the edges must be effected so as to prevent lubricating oil from getting out to the edges where it is thrown off by centrifugal force onto surrounding parts and where it can migrate around the edge onto the surface of the belt.
Another difficulty which is inherent in the operation of the mechanism is that the width of the belt for optimum design is wider than the shoe. This means that the portion of the flexible belt which passes beneath the shoe is compressed and is of less thickness as it emerges from beneath the shoe as compared with the portions of the belt on each side of the shoe that have not been compressed. This difference in thickness caused by the compression plus the nonuniform density of the lubricating oil across the face of the belt at the edge of the shoe makes it difficult to apply a simple removal element which treats the belt uniformly across its entire width. In other words, while the belt recovers its thickness after it passes out from beneath the shoe, at high speeds this recovery occurs after the belt has traveled some distance beyond the trailing edge of the shoe. Also, the lubricating oil which is applied between the belt and shoe must be essentially uniform across the entire width of the shoe face and yet a minimum amount of lubricating oil should be present beyond the edges of the shoe to avoid having excess oil which will fly off the belt and tend to travel outwardly to the belt edge where it can get onto the other face of the belt and contaminate the web and felts. It is also possible that a variation in viscosity can occur in the lubricating oil due to the heat generated in the lubricating oil as it passes beneath the shoe as contrasted with the oil at the edge which is not compressed between the shoe and the belt.
It is accordingly an object of the present invention to provide a method and mechanism for the removal and control of lubricating oil used to provide a hydraulic lubrication film between the shoe and belt of an extended nip press.
A further object of the invention is to provide an improved method and mechanism which permits operation of an extended nip press at high speeds and prevents the migration and escape of lubricating oil to other parts of the machine and to the edges of the belt and around the edges onto the surface of the belt which carries the felts and the web.
Other objects, advantages and features as well as equivalent methods and structures which are intended to be covered herein will become more apparent with the teaching of the principles of the present invention in connection with the disclosure of the preferred embodiments in the specification, claims and drawings in which: